Add default hierarchy restrictions

What does this MR do and why?

Fills hierarchy restrictions for objective/key result:

• tasks can be assigned to issues
• objectives can be assigned to other objectives (max 9 levels of nesting)
• key results can be assigned to objectives

These rules are not used yet, but will be used later in a follow-up.

Related to #378801 (closed)

Screenshots or screen recordings

DB migration:

$ rake db:migrate:main db:migrate:ci VERSION=20221116143854
main: == 20221116143854 AddOkrHierarchyRestrictions: migrating ======================
main: == 20221116143854 AddOkrHierarchyRestrictions: migrated (0.0189s) =============

ci: == 20221116143854 AddOkrHierarchyRestrictions: migrating ======================
ci: -- The migration is skipped since it modifies the schemas: [:gitlab_main].
ci: -- This database can only apply migrations in one of the following schemas: [:gitlab_ci, :gitlab_shared, :gitlab_internal].
ci: == 20221116143854 AddOkrHierarchyRestrictions: migrated (0.0001s) =============


$ rake db:rollback:main db:rollback:ci
main: == 20221116143854 AddOkrHierarchyRestrictions: reverting ======================
main: == 20221116143854 AddOkrHierarchyRestrictions: reverted (0.0057s) =============

ci: == 20221116143854 AddOkrHierarchyRestrictions: reverting ======================
ci: -- The migration is skipped since it modifies the schemas: [:gitlab_main].
ci: -- This database can only apply migrations in one of the following schemas: [:gitlab_ci, :gitlab_shared, :gitlab_internal].
ci: == 20221116143854 AddOkrHierarchyRestrictions: reverted (0.0002s) =============

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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